One time programmable memory and method of operation
View Patent ↗A one time programmable (OTP) memory has two-bit cells for increasing density. Each cell has two select transistors and a programmable transistor in series between the two select transistors. The programmable transistor has two independent storage locations. One is between the gate and a first source/drain region and the second is between the gate and a second source/drain region. The storage locations are portions of the gate dielectric where the sources or drains overlap the gate and are independently programmed by selectively passing a programming current through them. The programming current is of sufficient magnitude and duration to permanently reduce the impedance by more than three orders of magnitude of the storage locations to be programmed. The programming current is limited in magnitude to avoid damage to other circuit elements and is preferably induced at least in part by applying a negative voltage to the gate of the programming transistor.
1. A one time programmable (OTP) memory having a two-bit memory cell, wherein the two-bit memory cell comprises:
a first select transistor having a first current electrode coupled to a bit line, a control electrode coupled to a first word line, and a second current electrode;
a programmable transistor having a first current electrode coupled to the second current electrode of the first select transistor, a control electrode dielectric, a control electrode coupled to a programming line, and a second current electrode; and
a second select transistor having a first current electrode, a control electrode coupled to a second word line, and a second current electrode coupled to the second current electrode of the programmable transistor,
wherein the programmable transistor has a first programmable region between the first current electrode thereof and the control electrode thereof and a second programmable region between the second current electrode thereof and the control electrode thereof, wherein in response to the first select transistor being conductive, current is passed through the first current electrode of the programmable transistor and the control electrode dielectric to the control electrode thereof in a region where the control electrode and the first current electrode overlap, and in response to the second select transistor being conductive, current is passed through the second current electrode of the programmable transistor and the control electrode dielectric to the control electrode thereof in a region where the control electrode and the second current electrode overlap, wherein current flows from the current electrode of the programmable transistor during a program operation.
2. The OTP memory of claim 1 wherein the the first current electrode of the second select transistor is coupled to the bit line wherein both the first current electrode of the first select transistor and the first current electrode of the second select transistor are coupled to the same bit line.
3. The OTP memory of claim 1 wherein the first and second programmable regions are converted from an initial impedance to a relatively lower impedance responsive to application of a negative voltage applied to the control electrode of the programmable transistor.
4. The OTP memory of claim 1 wherein the first programmable region is converted from an initial impedance to a relatively lower impedance by flowing current through the first and second current electrodes of the first select transistor, the first current electrode of the programmable transistor, and the control electrode of the programmable transistor.
5. The OTP memory of claim 1 wherein the second programmable region is converted from an initial impedance to a relatively lower impedance by flowing current through the first and second current electrodes of the second select transistor, the second current electrode of the programmable transistor, and the control electrode of the programmable transistor.
6. The OTP memory of claim 1 further comprising:
word line select circuitry having a first output coupled to the control electrode of the first select transistor, a second output coupled to the control electrode of the second select transistor, and a third output; and
current limiting circuitry having an input coupled to the third output of the word line select circuitry and an output coupled to the control electrode of the programmable transistor.
7. The OTP memory of claim 1 further comprising a plurality of two-bit memory cells coupled to the bit line.
8. The OTP memory of claim 1 further comprising a plurality of two-bit memory cells coupled to the first and second word lines and the programming line.
9. The OTP memory of claim 1 further comprising a semiconductor substrate, wherein the second current electrode of the first select transistor and the first current electrode of the programmable transistor share a doped region in the semiconductor substrate.
10. The OTP memory of claim 1 , wherein the first programmable region and the second programmable region can independently be converted from an initial impedance to a lower impedance wherein the initial impedance is more than three orders of magnitude greater than the relatively lower impedance.
11. A method of operating a one time programmable (OTP) memory, comprising:
providing a first select transistor having a first current electrode coupled to a bit line, a control electrode coupled to a first word line, and a second current electrode;
providing a programmable transistor having a first current electrode coupled to the second current electrode of the first select transistor, a control electrode coupled to a programming line, and a second current electrode;
providing a second select transistor having a first current electrode coupled to the same bit line, a control electrode coupled to a second word line, and a second current electrode coupled to the second current electrode of the programmable transistor, the first select transistor, the programmable transistor and the second select transistor forming a two-bit memory cell;
programming a first bit by applying a first programming current through the first and second current electrodes of the first select transistor, the first current electrode of the programmable transistor, and the control electrode of the programmable transistor; and
programming a second bit by applying a second programming current through the first and second current electrodes of the second select transistor, the second current electrode of the programmable transistor, and the control electrode of the programmable transistor.
12. The method of claim 11 , wherein to program the first bit further comprises applying an enable signal to the first word line and a disable signal to the second word line.
13. The method of claim 11 , wherein to program the first bit further comprises enabling the first select transistor, disabling the second select transistor, and applying a voltage differential between the first current electrode of the first select transistor and the control electrode of the programmable transistor.
14. The method of claim 13 , wherein the voltage differential causes the first programming current to flow from the first current electrode of the programmable transistor to the control electrode of the programmable transistor.
15. The method of claim 14 , wherein the voltage differential comprises a negative voltage applied at the control electrode of the programmable transistor and a positive voltage applied at the first current electrode of the first select transistor.
16. The method of claim 15 , further comprising:
limiting the first programming current sufficiently to avoid damage to the first current electrode of the programmable transistor.
17. The method of claim 11 , wherein the first programming current is of sufficient magnitude and duration to cause a permanent reduction in an impedance between the first current electrode and the control electrode of the programmable transistor.
18. A one time programmable (OTP) memory cell comprising a programmable transistor in series with two select transistors between two contacts of a single bit line, wherein the programmable transistor comprises a first programmable region between a gate thereof and a first source/drain thereof and a second programmable region between the gate and a second source/drain thereof.
19. The one time programmable (OTP) memory cell of claim 18 , wherein the first programmable region is a first portion of a gate dielectric of the programmable transistor, the second programmable region is a second portion of the gate dielectric, and the first and second portions of the gate dielectric are permanently programmable to a condition of reduced impedance.
20. The one time programmable (OTP) memory cell of claim 19 , wherein the first and second portions of the gate dielectric are converted to the condition of reduced impedance by current flow therethrough.